A variety of applications require the use of a uniform line image formed from a high-power laser beam. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA) or just “laser annealing,” which is used in semiconductor manufacturing to activate dopants in select regions of a semiconductor wafer when forming active microcircuit devices such as transistors.
One type of laser annealing uses a scanned line image formed from a laser beam to heat the surface of the semiconductor wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants but short enough to minimizing dopant diffusion. The time that the surface of semiconductor wafer is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
One type of high-power laser that is used for laser annealing applications is CO2 laser operating in a single spatial mode regime. Traditional methods of performing laser annealing with a CO2 laser including illuminating a pair of knife-edges by collimated CO2 laser beam and then relaying the light passing therethrough to an image plane to form the line image (imaging knife edges onto the semiconductor wafer).
FIG. 1 is a plot of the intensity profile I(x) and the thermal emission profile E(x) versus x (mm) as for a line image as measured at an image plane of a line-forming optical system. The x-direction is the long direction of the line image. The thermal emission profile E(x) is the measured thermal emission from a semiconductor wafer located at the image plane when irradiated with the line image having the intensity profile I(x). As can best be seen in the thermal emission profile E(x), intensity non-uniformities that show up as ripples in the intensity profile I(x) of line image cause non-uniformities in the heating of the semiconductor wafer. The intensity non-uniformities in the intensity profile I(x) are due to diffraction effects caused by the limited apertures of mirrors in the line-forming optical system.
What is needed is a way to smooth out the intensity ripples without substantially impacting the overall amount of intensity delivered to the image plane.